Perennial herbs, glabrous, aromatic, growing in marshes or as emergent aquatics; laticifers and raphides absent; aerenchyma present. Rhizome creeping, much branched, lacunose, with specialized aromatic oil cells; roots on lower side and bearing leaves at apex. Leaves distichous, bases overlapping, unifacial, ensiform, not differentiated into petiole and blade; intravaginal squamules present in leaf axils; venation parallel. Inflorescence solitary, terminal, borne laterally on leaflike scape (usually interpreted as peduncle and spathe; peduncle with 2 separate vascular systems); continuous shoot arising in axil of leaf preceding inflorescence; spathe much longer than spadix, erect, persistent (appearing merely as a vertical extension of leaflike peduncle); spadix jutting out at an angle from peduncle, sessile, conoid, cylindric and fingerlike or slender and tail-like, flowering from base to top. Flowers bisexual, with perigone, densely arranged, bractless, 3-merous; tepals 6, in 2 whorls of 3, free, thin, arched; stamens 6, in 2 whorls of 3, free, filaments linear-oblong and flattened, anthers introrse, thecae globose-ellipsoid, subopposite, dehiscing by longitudinal slit, connective inconspicuous; pollen monosulcate, ellipsoid, small (15-20 µm), exine shallowly and remotely or more densely foveolate, otherwise psilate, apertural exine subpsilate; gynoecium (pistil) obconic-cylindric, only slightly exceeding tepals, ovary 2- or 3-locular, ovules several per locule, orthotropous (atropous), pendent on apical placenta, both integuments bearing trichomes and inner integument longer than outer, forming micropyle, stigma minute (± punctate), subsessile (a broad stylar region especially seen by longitudinal cut). Fruit a few-seeded berry, oblong-obovoid with thinly leathery pericarp, enclosed by tepals, ± whitish with brownish stigma remnant when fresh, soon drying to straw-brown, 1-5(-9)-seeded. Seed oblong to ellipsoid; testa light brown, with small pits (slightly foveolate) (Acorus calamus) or smooth (A. gramineus), long integumentary trichomes (bristles) present at micropyle (A. gramineus) or absent (A. calamus); embryo axile, cylindric or conoid (A. gramineus), with perisperm and abundant endosperm. 2n = (22), 24, 36, (44), 48.
Herbs, perennial, wetland, usually with aromatic oil, especially in rhizomes. Rhizomes horizontal, creeping at or near surface, branched. Stems repent, branched rhizomes. Cataphylls absent. Leaves not differentiated into petiole and blade, equitant, sword-shaped, larger than 1.5 cm; venation parallel along length of leaf. Inflorescences: spadices, from 3-angled axis (peduncle fused with proximal portion of sympodial leaf, i.e., leaf encircling terminal inflorescence), distal sympodial leaf extending beyond spadix; true spathe absent; spadix nearly cylindric, tapering, apex obtuse. Flowers bisexual; tepals 6; stamens 6, distinct; ovariesy 1, (1--)3-locular, sessile; stigmas sessile (styles essentially absent), minute. Fruits berries; pericarp thin, leathery. Seeds 1--6(--14), from apex of locule.
Grayum, M. H. 1987. A summary of evidence and arguments supporting the removal of Acorus from the Araceae. Taxon 36: 723--729. Thompson, S. A. 1995. Systematics and Biology of the Araceae and Acoraceae of Temperate North America. Ph.D. dissertation. University of Illinois. Urbana-Champaign.